This invention relates to direct downconversion receivers and more particularly to a direct downconversion receiver architecture that mitigates the DC offset component.
Conventional radio receivers typically employ a superheterodyne architecture. In such architecture, the receiver down converts the received RF signal to an intermediate frequency (IF). After additional processing, the IF signal is then converted to a baseband signal. In contrast, a direct down conversion receiver translates the received RF signal to baseband in just one stage of processing. Because direct down conversion receivers do not translate the received RF signal to an intermediate frequency, they are also known as xe2x80x9czero-IFxe2x80x9d receivers. Given that each processing stage in a receiver inevitably introduces noise and requires more components, much effort has been directed to designing direct down conversion receivers.
Despite the attractive signal-to-noise and manufacturing advantages of a direct down conversion receiver, most applications such as wireless handsets continue to use a superheterodyne architecture because of DC offset problems arising from local oscillator (LO) self-mixing, LO coupling back to a low noise amplifier (LNA), mixer 2nd order distortion, and envelope detection of interfering AM signals. Also significant issues arise from the contribution of LO 1/f noise, reciprocal mixing, and LO spurious signals.
LO self-mixing arises in a direct down conversion receiver as follows. The mixer receives in one port the received RF signal at a carrier frequency and receives in the other port the LO signal also at or near the carrier frequency. This local oscillator signal is typically much higher power than the received RF signal and inevitably reactively couples into the RF port and thus self mixes. In this self-mixing process, a sinusoid such as the LO signal is squared and produces a DC offset component. Such a DC offset component at baseband will interfere with the demodulation of digital signals, particularly at the higher throughputs common in modern digital communication systems.
Accordingly, there is a need in the art for an improved direct down conversion receiver architecture that alleviates the DC offset problem.
In accordance with a first aspect of the invention, a direct downconversion receiver includes an oscillator for providing a local oscillator signal. A phase-shifter is configured to receive the LO signal and provide a phase-shifted signal that is approximately 90 degrees out of phase with the LO signal. The receiver combines the LO signal with an incoming RF signal and provides the combined signal to a first input port of a mixer. The mixer includes a second port for receiving the phase-shifted signal such that the mixer provides a baseband output signal that is the product of the signals entering the first input port and the second input port.
In accordance with another aspect of the invention, a method is provided to directly downconvert an RF signal. In this method, the RF signal is combined with an LO signal to produce a combined signal. The LO signal is also phase-shifted by approximately 90 degrees to provide a quadrature signal. The quadrature signal and the combined signal are multiplied together to produce a baseband output signal.
The invention will be more fully understood upon consideration of the following detailed description, taken together with the accompanying drawings.